It never really made sense in the game either since Salt Lake had a lot of infected including Bloaters. Reality is the fungus is whatever it needs to be to develop and pace the story, that goes for both mediums. I really dont stress about it too hard as long as the show is still entertaining and well acted which I find it to be.
Realistically, if the fungus developed a way to take over humans, it could do so anywhere I suppose but it would really only spread effectively in warmer climates, so yea a human could still incubate it in the far north if bit, but you are less likely to find colonies of the spore like they showed in Boston museum.
Really they treat them more like zombies with nothing being airborne, it makes it work better for a tv show you dont want actors in gas masks. Also that way they can be anywhere at any time.